It is well known that a lower olefin and a lower aliphatic carboxylic acid react in the presence of an acid catalyst to provide the corresponding ester. It is also known that in this reaction, a heteropolyacid and/or heteopolyacid salt effectively act as a catalyst. Specific examples of these conventional techniques include those described, for example, in Japanese Unexamined Patent Publications No. 4-139148, No. 4-139149, No. 5-65248, No. 5-163200, No. 5-170699, No. 5-255185, No. 5-294894, No. 6-72951 and No. 9-118647. Thus, the development of catalysts having high initial activities is proceeding.
However, in industrial production processes, impurities derived from the starting materials or by-products produced during the reaction cause deterioration of the catalyst and in turn the reaction yield disadvantageously decreases. Particularly, in the process having a circulation system, if a reaction is continuously performed for a long time, various impurities or byproducts accumulate in the system and due to the effect thereof, for example, a vicious circle arises such that the catalyst deteriorates and the side reaction is further accelerated.